In the 89th minute of Saturday's game at AAMI Park, which Brisbane lost to Melbourne Victory 2-0, Yallop limped off and was treated behind the goal as she clutched at her right hamstring.
The midfielder tried to hobble to the bench with help, but couldn't put any weight on her right leg and was ultimately piggy-backed to the change rooms.
While Brisbane could not make finals, a serious injury would rule Yallop, who has been in dazzling form and scored 12 goals this season, out of Australia's May and June friendlies against Argentina.
Yallop's injury comes just three days after confirmation of Mary Fowler's devastating ACL tear.
"I think it's her hammy," Roar coach Alex Smith said of Yallop.
"So we'll get back to Brisbane, she'll get a scan on Monday or Tuesday, and then we'll go from there. Until we get that, we don't really know any more.
"She said it locked out a little bit, so we just need to figure out why that is, and if it's bad, it's bad, and it just kind of sums up the game, sums up 2025 for our football club.
"But we'll just have to wait and see."
The game was a dead rubber because Victory could not leapfrog premiers Melbourne City, nor be overtaken in second place, while seventh-placed Brisbane had already missed finals.
Victory coach Jeff Hopkins rested striker Emily Gielnik (12 goals), which effectively confirmed City star Holly McNamara (15 goals) won the golden boot.
Victory, who have won seven straight games and are unbeaten in 13, have next week off, with Gielnik and defender Ellie Wilson saved for the following week's semi-final first leg.
"If we would have been playing for a premiership today, both probably would have played," Hopkins said.
"We wrapped her up in cotton wool today ... I had to make a decision, and I made that decision to look after her for the bigger games."
Nickoletta Flannery opened the scoring in the 43rd minute with a wonderful strike into the top corner.
Alex Chidiac nutmegged Roar goalkeeper Keeley Richards for Victory's second in the 54th.
Brisbane's Momo Hayashi was sent off in the 60th minute, and Smith indicated the Roar would appeal the Japanese midfielder's red card.
"Apparently, at least what I've been told, it was contact with the referee," he said.
"And if anybody knows Momo, she's the nicest, kindest soul you'll ever meet and barely speaks English, and doesn't get angry at anybody.
"So I haven't seen it, but I'd trust someone like Momo with my children, and she says it wasn't much.
"I'm fairly confident it'll get overturned on appeal just because of the player it is."
Newcastle host Central Coast and Western Sydney play Canberra later on Saturday.
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